GROUNDWATER FLOW TO THE COASTAL OCEAN
A. E. Mulligan and M. A. Charette, Woods Hole
Oceanographic Institution, Woods Hole, MA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
Water flowing through the terrestrial landscape ultimately delivers fresh water and dissolved solutes to
the coastal ocean. Because surface water inputs (e.g.,
rivers and streams) are easily seen and are typically
large point sources to the coast, they have been well
studied and their contributions to ocean geochemical
budgets are fairly well known. Similarly, the hydrodynamics and geochemical importance of surface
estuaries are well known. Only recently has significant attention turned toward the role of groundwater
inputs to the ocean. Historically, such inputs were
considered insignificant because groundwater flow is
so much slower than riverine flow. Recent work
however has shown that groundwater flow through
coastal sediments and subsequent discharge to the
coastal ocean can have a significant impact on geochemical cycling and it is therefore a process that
must be better understood.
Groundwater discharge into the coastal ocean generally occurs as a slow diffuse flow but can be found as
large point sources in certain terrain, such as karst. In
addition to typically low flow rates, groundwater
discharge is temporally and spatially variable, complicating efforts to characterize site-specific flow regimes. Nonetheless, the importance of submarine
groundwater discharge (SGD) as a source of dissolved
solids to coastal waters has become increasingly recognized, with recent studies suggesting that SGDderived chemical loading may rival surface water
inputs in many coastal areas. So while the volume of
water discharged as SGD may be small relative to
surface discharge, the input of dissolved solids from
SGD can surpass that of surface water inputs. For
example, SGD often represents a major source of nutrients in estuaries and embayments. Excess nitrogen
loading can result in eutrophication and its associated
secondary effects including decreased oxygen content,
fish kills, and shifts in the dominant flora.
First, let us define ‘groundwater’ in a coastal
context. We use the term to refer to any water
that resides in the pore spaces of sediments at the
land–ocean boundary. Hence, such water can be
fresh terrestrially derived water that originates as
rainwater infiltrating through the subsurface or it can
represent saline oceanic water that flows through the
sediments (Figure 1). Therefore, groundwater discharging to coastal waters can have salinity that
spans a large range, being some mixture of the two
end members. We therefore use the terms fresh SGD
and saline SGD to distinguish these sources of fluid
and brackish SGD to mean a mixture of the fresh and
saline end members.
Basics of Groundwater Flow
Groundwater flow in the subsurface is driven by
differences in energy – water flows from high energy
areas to low energy. The mechanical energy of a unit
volume of water is determined by the sum of gravitational potential energy, pressure energy, and kinetic
energy:
Energy per unit volume ¼ rgz þ P þ
rV
2
2
½1
where r is fluid density, g is gravitational acceleration, z is elevation of the measuring point relative to
a datum, P is fluid pressure at the measurement
point, and V is fluid velocity. Because groundwater
flows very slowly (on the order of 1 m day
À 1 or less),
its kinetic energy is very small relative to its gravitational potential and pressure energies and the
kinetic energy term is therefore ignored. By removing
the kinetic energy term and rearranging eqn [1] to
express energy in terms of mechanical energy per
unit weight, the concept of hydraulic head is developed:
Energy per unit weight ¼ hydraulic head ¼ z þ
P
rg
½2
Groundwater therefore flows from regions of high
hydraulic head to areas of low hydraulic head.
Because groundwater flows through a porous
media, the rate of flow depends on soil properties
such as the degree to which pore spaces are interconnected. The property of interest in groundwater
flow is the permeability, k, which is a measure of the
ease with which a fluid flows through the soil matrix.
Groundwater flow rate can then be calculated using
Darcy’s law, which says that the flow rate is linearly
proportional to the hydraulic gradient:
q ¼ À
rgk
m
ðrhÞ
½ 3
465
A. E. Mulligan and M. A. Charette, Woods Hole
Oceanographic Institution, Woods Hole, MA, USA
& 2009 Elsevier Ltd. All rights reserved.
Introduction
Water flowing through the terrestrial landscape ultimately delivers fresh water and dissolved solutes to
the coastal ocean. Because surface water inputs (e.g.,
rivers and streams) are easily seen and are typically
large point sources to the coast, they have been well
studied and their contributions to ocean geochemical
budgets are fairly well known. Similarly, the hydrodynamics and geochemical importance of surface
estuaries are well known. Only recently has significant attention turned toward the role of groundwater
inputs to the ocean. Historically, such inputs were
considered insignificant because groundwater flow is
so much slower than riverine flow. Recent work
however has shown that groundwater flow through
coastal sediments and subsequent discharge to the
coastal ocean can have a significant impact on geochemical cycling and it is therefore a process that
must be better understood.
Groundwater discharge into the coastal ocean generally occurs as a slow diffuse flow but can be found as
large point sources in certain terrain, such as karst. In
addition to typically low flow rates, groundwater
discharge is temporally and spatially variable, complicating efforts to characterize site-specific flow regimes. Nonetheless, the importance of submarine
groundwater discharge (SGD) as a source of dissolved
solids to coastal waters has become increasingly recognized, with recent studies suggesting that SGDderived chemical loading may rival surface water
inputs in many coastal areas. So while the volume of
water discharged as SGD may be small relative to
surface discharge, the input of dissolved solids from
SGD can surpass that of surface water inputs. For
example, SGD often represents a major source of nutrients in estuaries and embayments. Excess nitrogen
loading can result in eutrophication and its associated
secondary effects including decreased oxygen content,
fish kills, and shifts in the dominant flora.
First, let us define ‘groundwater’ in a coastal
context. We use the term to refer to any water
that resides in the pore spaces of sediments at the
land–ocean boundary. Hence, such water can be
fresh terrestrially derived water that originates as
rainwater infiltrating through the subsurface or it can
represent saline oceanic water that flows through the
sediments (Figure 1). Therefore, groundwater discharging to coastal waters can have salinity that
spans a large range, being some mixture of the two
end members. We therefore use the terms fresh SGD
and saline SGD to distinguish these sources of fluid
and brackish SGD to mean a mixture of the fresh and
saline end members.
Basics of Groundwater Flow
Groundwater flow in the subsurface is driven by
differences in energy – water flows from high energy
areas to low energy. The mechanical energy of a unit
volume of water is determined by the sum of gravitational potential energy, pressure energy, and kinetic
energy:
Energy per unit volume ¼ rgz þ P þ
rV
2
2
½1
where r is fluid density, g is gravitational acceleration, z is elevation of the measuring point relative to
a datum, P is fluid pressure at the measurement
point, and V is fluid velocity. Because groundwater
flows very slowly (on the order of 1 m day
À 1 or less),
its kinetic energy is very small relative to its gravitational potential and pressure energies and the
kinetic energy term is therefore ignored. By removing
the kinetic energy term and rearranging eqn [1] to
express energy in terms of mechanical energy per
unit weight, the concept of hydraulic head is developed:
Energy per unit weight ¼ hydraulic head ¼ z þ
P
rg
½2
Groundwater therefore flows from regions of high
hydraulic head to areas of low hydraulic head.
Because groundwater flows through a porous
media, the rate of flow depends on soil properties
such as the degree to which pore spaces are interconnected. The property of interest in groundwater
flow is the permeability, k, which is a measure of the
ease with which a fluid flows through the soil matrix.
Groundwater flow rate can then be calculated using
Darcy’s law, which says that the flow rate is linearly
proportional to the hydraulic gradient:
q ¼ À
rgk
m
ðrhÞ
½ 3
465